The UN's top rights body will on Wednesday debate a draft resolution condemning the Syrian regime's use of foreign fighters in the besieged town of Qusayr.
A draft resolution presented on Tuesday "condemns the intervention of foreign combatants fighting on behalf of the Syrian regime in Al-Qusayr," an implicit reference to the involvement of fighters from Lebanon's militant group Hezbollah in the fierce battle for the strategic town.
The draft resolution, which was put forward by the United States, Turkey and Qatar and which if passed would be non-binding, cautions that the presence of the foreign fighters in the western town "poses a serious threat to regional stability".
The text also calls on the Syrian authorities to "allow free and unimpeded access by the United Nations and humanitarian agencies to all civilians affected by the violence, especially in Al-Qusayr."
Hundreds of civilians have reportedly been killed in the rebel stronghold since the regime began its assault on 19 May , while thousands could remain trapped there.
The draft resolution stressed "the need to ensure accountability for those responsible for the massacre in Al-Qusayr, (as well as for) all those responsible for the serious violations of international humanitarian rights law in Syria."
Hezbollah has lost dozens of fighters in its bid to help the Syrian regime regain control of the town near the Lebanese border, and its involvement is threatening to draw Lebanon ever deeper into the Syria conflict, raising domestic tensions.
On Monday, rockets fell for the first time in the southern suburbs of Beirut, a stronghold of the Shiite movement, and gunmen killed three Lebanese soldiers near the Syrian border overnight to Tuesday.
The Syrian crisis is also spreading in northern Lebanon, where fighting between Sunnis and Alawites, the sect to which Assad belongs, killed 31 people last week.
Hezbollah's involvement in the fight has already stirred international condemnation, with UN chief Ban Ki-moon saying he was "deeply concerned" by the group's role.